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The first is diazotised and combined or coupled with the second.
The coupling takes place, in the case of amines, in a faintly
acid or neutral solution, in the case of phenols, in an alkaline
solution (see Reaction 6, p. 163). In all cases the diazo-group
seizes upon the carbon in the para-position to the ammo- or
hydroxyl group of the coupled nucleus. When the para-position
is already appropriated, the ortho-position serves as a link, but
no coupling ever occurs in the meta-position. The sulphonic
acid derivatives of the base or phenol are frequently preferable
to the unsubstituted compound. The dyes formed have in
consequence of the presence of the SO3H group an acid character,
which renders them capable of forming soluble sodium salts,
and adapts them better for dyeing purposes. When an azo-com-
pound is formed by coupling the diazo-compouncl with a primary
amine, the new product is capable of being diazotised and
coupled a second time. Thus a tetrazo-compound is formed
containing a double diazo-group ~N:N~. Aminoazobenzene,
when diazotised, forms diazo-azobenzene with nitrous acid, which,
like a simple diazo-compound, reacts with the phenols,
C6H5N:N.CHH4NILHC1 + UNO, = C0I-I5N:N.CuH4N.j.Cl -I- 2lIaO.
CttH5N:N.C6H4No.Cl + C0II5ONa
If aminoazobenzene is sulphonated with fuming sulphuric acid,
and the product again diazotised and coupled with /3-naphthol,
Biebrich scarlet is formed,
C6H4<                 /SO,II
Biebrich scarlet.
If in the last phase the different sulphonic acids of /3-naphthol
are employed, various shades of red, known as Crocciiis^ are pro-
duced. Thus it appears that the colour deepens from orange to
red with the introduction of a second azo-group.
This is not the only method of forming tetrazo-compounds.
Each ammo-group of a diamine may be diazotised and coupled.
Benzidine and its homologues, which have been utilised in this
way, have a special value for the cotton dyer, as the shades pro-
duced are not only very brilliant, but, unlike the majority of